1. Access Methods File store information When it is used it is accessed & read into memory Some systems provide only one access method IBM support many access methods Choosing right one for a particular application

2. Access Methods Sequential Access read next write next reset no read after last write (rewrite) Direct Access read n write n position to n read next write next rewrite n n = relative block number

3. Sequential-access File  Simplest access method  Information accessed- one record after the other  Common method – ex: editors & compilers  Bulk of operations in file is read & write  Read – reads the next portion of the file & advances file pointer  Write – appends the end of the file & advances to the newly written material  Beginning – reset  Forward & backward n records – skip  Sequential access is based on the tape model of a file

4. Sequential-access File

5. Simulation of Sequential Access on a Direct-access File

6. Direct Access (relative access)  file – fixed length, logical records (allow programs to read & write rapidly), no particular order  Random access to any file block  Based on disk model of a file  File is viewed as a numbered sequence of blocks / records  No restrictions – read 24, read 54, write 5  Block number provided by user to OS is normally a relative block number  It is an index relative to index of a file  Relative blocks – 0,1,… / 1,………..  Use – allows OS to decide where the file should be placed &helps users from accessing file part that is not a part of it  Logical record length – L request for record – N turns to I/O request for L bytes starting from location L*(N-1) within the file  Logical records are of fixed size – easy to read, write & delete a record

7. Other access methods  They can built on the top of the direct access method  Involve the construction of index for a file  Contains pointers for various blocks  Find a record in a file – search the index – use the pointer to access the file directly – find the desired record  Ex: retail price file  Large files – index file will be large to be to kept in memory  Solution – create an index for the indexed file  Primary index file contain pointers to secondary index file which would point actual data item  Ex: IBM’s indexed sequential access method (ISAM) uses small master index for secondary index  Find a item: binary search in master index for finding the block number in secondary index – block is read – again binary search to find the block containing the desired record – block is searched sequentially  Any records can be accessed by this two direct access reads

8. Example of Index and Relative Files

9. Directory Structure A collection of nodes containing information about all files F 1 F 2 F 3 F 4 F n Directory Files Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes

10. The file systems of computer can be extensive Store – millions of files (TB), manage – organize Organization – 2 parts: Disk split into 1/more partitions – minidisks Partition – low level structure – files & directories reside Partition contains information about files in it Information is kept in device directory / volume table of contents Device dir – records name, location, size, type Directory – symbol table – translates file name into directories Insert, delete, search, list all entries

11. A Typical File-system Organization

12. Operations Performed on Directory Search for a file – dir struct – entry for a file Create a file – new file created & added to dir Delete a file –no longer needed remove from directory List a directory- list all the files Rename a file – changing the name Traverse the file system – checking the entire directory

13. Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users Two users can have same name for different files The same file can have several different names Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

14. Single-Level Directory A single directory for all users Naming problem Grouping problem

15. Two-Level Directory Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability

16. Tree-Structured Directories

17. Tree-Structured Directories (Cont) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list

18. Tree-Structured Directories (Cont) Absolute or relative path name Creating a new file is done in current directory Delete a file rm Creating a new subdirectory is done in current directory mkdir Example: if in current directory /mail mkdir count mail progcopyprtexpcount Deleting “mail”  deleting the entire subtree rooted by “mail”

19. Acyclic-Graph Directories Have shared subdirectories and files

20. Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list  dangling pointer Solutions: Backpointers, so we can delete all pointers Variable size records a problem Backpointers using a daisy chain organization Entry-hold-count solution New directory entry type Link – another name (pointer) to an existing file Resolve the link – follow pointer to locate the file

21. General Graph Directory

22. General Graph Directory (Cont.) How do we guarantee no cycles? Allow only links to file not subdirectories Garbage collection Every time a new link is added use a cycle detection algorithm to determine whether it is OK